1. Field of the Invention
The present invention relates to an identification information recording apparatus and an identification information recording method for an optical disk that records on an optical disk such as Blu-ray Disk or HD-DVD identification information for discriminating an optical disk from another optical disk.
2. Description of the Related Art
Conventionally, identification information (media ID) specific to every optical disk is recorded on an optical disk such as Blu-ray Disk or HD-DVD in order to manage the optical disk during its manufacture and storage or during its distribution and sale. In general, the identification information is represented by a barcode and recorded to a BCA (Burst Cutting Area) or NBCA (Narrow Burst Cutting Area) formed at the innermost peripheral portion of the optical disk.
The identification information is recorded such that an elongated optical spot is formed on the BCA (or NBCA) of the optical disk along the radial direction of the optical disk for melting to remove the reflection layer formed on the optical disk. In an identification information recording apparatus disclosed in JP2005-11435A, for example, as shown in FIG. 9A, laser beam is irradiated in the direction vertical to the optical disk to form an elongated optical spot on the optical disk, wherein the optical spot is moved toward the outer side from the inner side of the optical disk with the edge portions of the optical spot in the longitudinal direction overlapped with each other, thereby forming the identification information represented by a barcode at the BCA (or NBCA) of the optical disk. In this case, as shown in FIG. 9B, the light intensity distribution of the elongated optical spot is such that the light intensity at the front side in the moving direction of the optical spot is set higher than the light intensity at the rear side in the moving direction. The reason why the light intensity at the front side in the moving direction of the optical spot is set higher is that the generation of semi-molten articles or molten residues of the reflection layer, which is likely to be produced at the front side in the moving direction, is prevented in order to record the barcode with high precision.
However, as a result of the experiment, carried out by the present inventor, for recording a barcode on an optical disk with the above-mentioned light intensity distribution of the optical spot, stripes are generated in the radial direction of the optical disk, and hence, the barcode cannot precisely be recorded. The guess of the result of this experiment is as follows. The reflection layer of the optical disk on which the optical spot is formed is melted by the heat of the optical spot, and removed around the optical spot. In this case, as shown in FIG. 10A, the light intensity at the front side in the moving direction of the optical spot is high, whereby the reflection layer of the optical disk at the front side in the moving direction is rapidly melted to greatly rise upward due to the surface tension. After being cooled, the rising solid portions remain. As shown in FIG. 10B, the optical spot moves to the outer side of the optical disk, and a part of the rear side of the optical spot in the moving direction is formed on the rising solid portion, but the rising solid portion is not perfectly melted and remains unremoved since the light intensity at the rear side of the optical spot in the moving direction is low.
Therefore, the rising solid portion remains as a molten residue at the BCA (or NBCA) of the optical disk along its circumferential direction. This molten residue appears as a stripe pattern along the radial direction of the optical disk. This deteriorates the reproducing precision of a signal read from the BCA (or NBCA) of the optical disk, thereby entailing a problem that the identification information recorded to the BCA (or NBCA) cannot correctly be reproduced.